Class AB amplifiers are typically used for providing the amplified output required for loudspeakers, for such applications as home entertainment systems. Usually, the loudspeaker (hereinafter also referred to as the speaker) is connected to the amplifier via a capacitor This is called AC-coupled, since the DC is blocked by the capacitor. The speaker can also be directly coupled to the speaker. This is called DC-coupled and requires a differential or true ground connection. The true ground connection requires negative voltages such that the amplifier output signals are referenced to 0V (ground).
A typical such AC-coupled amplifier arrangement is shown in FIG. 1. Speaker 11 is connected via capacitor 12 to the output of an operational amplifier 14, having a feedback loop via feedback resistor 15 to one (−) input of the operational amplifier. An input signal 16 is connected, via an input resistor 17, across the two inputs (− and +) of the operational amplifier. The amplifier is powered at the supply voltage Vdd, and the input (+) held at half the supply voltage (ie at ½Vdd).
When such an amplifier is switched on, the capacitor 12 will be charged until the voltage across it becomes ½Vdd. With a typical capacitor value of 220 μF and a speaker impedance of 8Ω, the charging (to 3τ) takes about 5.3 ms. This results in an audible plop from the speaker. Depending on the shape of the voltage curve across the speaker the timbre of the plop varies. When the shape has sharp edges, such as does a square wave, higher harmonics are present.
To date, conventional systems use a relay with the power amplifiers in order to reduce or eliminate the plop. However relays are bulky and heavy, and as such are particularly inconvenient for use with headphones or earphones and speakers.
United States patent application, publication number U2006/0182265, discloses an amplifier arrangement having a main output stage and a switchably connectable low output stage. A sample-and-hold circuit is used to control the main output stage to reduce the plop. A single feedback loop is provided to the low output stage.